The present invention relates generally to optical filters and more particularly to an optical filter for use with an imaging lens in an incoherent imaging system for increasing the depth of focus of the imaging lens while maintaining energy thru-put. The invention is especially useful with but not exclusively limited to use with incoherent imaging systems involving phase conjugation for use in correcting window aberrations in aircraft, missiles or other sensor platforms.
The aerodynamic performance requirements for aircraft, missiles or other sensor platforms often necessitate the use of conformal windows that have curved surfaces which are not symmetrical. In addition, these windows often exhibit localized manufacturing errors, such as thickness and curvature variations as well as prismatic or wedge effects. These imperfections and asymmetries result in complex distortions of a scene when it is viewed through such windows. Some of the distortions may be characterized as follows: (1) localized or global shape distortions, (2) variable or localized magnification caused by the variations in optical power of the window, (3) one-dimensional magnifications and shifts caused by unequal curvatures of the window in different directions, and (4) other distortions caused by geometrical and/or surface defects on the outside surface of the window, the inside surface of the window or both the outside and inside surfaces of the window.
Sometimes the distortion caused by these irregularities is so severe that no recognizeable image can be obtained.
Therefore, errors of this magnitude may be devastating to the performance of an imaging sensor which may be placed behind the window. Similarly, these errors may significantly effect the judgement of a pilot viewing a scene through the window or sensor image because of binocular deviations or disparity. Because of these distortions, the design of windows on such vehicles, especially on high speed platforms used in conjunction with imaging sensors has, unfortunately, often been controlled primarily by the sensor imaging requirements, rather than by the aerodynamic requirements. In other instances the problem of image distortion caused by window aberrations has been avoided by simply eliminating the window element itself and viewing the scene to be observed through a hole in the aircraft or by using a flat window. Both of these solutions compromise the performance of the vehicle.
The process of phase conjugation, which results from multiplying a complex wavefront by its complex conjugate is a well-known technique for correcting wavefront distortion. Phase conjugation techniques using conjugate refractive (or reflective) type elements have also been used in a variety of circumstances to correct wavefront distortion. For example, in non-linear optics, a non-linear medium is used in conjunction with four-wave laser mixing to create real-time phase conjugated (corrected) wavefronts. Conjugate wavefronts have also been used to dynamically remove the effects of atmospheric turbulence from telescopes and other optical systems. In this type of application, referred to as "adaptive optics", interferometric measurements and appropriate servo-loops are used to drive flexible optical elements to dynamically realize phase conjugations, and thus wavefront correction.
A talk pertaining to broadband phase conjugation using thin flat plate aberrators and entitled Phase Conjugation With Incoherent Radiation was presented by G. O. Reynolds, D. A. Servaes and J. B. DeVelis on Oct. 19, 1982 at the 1982 annual meeting of the Optical Society of America and is summarized on page 1746 of the Journal of Optical Society of America, December, 1982, Volume 72. A talk entitled, "Phase Conjugation with Partially Coherent Radiation," was presented by D. A. Servaes, J. B. DeVelis and G. O. Reynolds, at the Optical Computing Conference, M.I.T. Boston, MA, on Apr. 7, 1983. In both of the above talks it was shown that a thin, flat glass plate randomly scratched on one side could be passively phase conjugated for use with white light (i.e. broadband radiation) by placing a nearly index matched plastic replica of that plate in the image plane of the plate formed by an imaging lens, thereby creating a substantially undistorted image of a target when viewed through the lens by another imaging system.
Recently, it has been suggested that broadband phase conjugation be used to correct complex phase aberrations in aerodynamic windows or other types of "thick" aberrators (i.e. transparent elements more than about a few millimeters thick). A system for accomplishing this could comprise a conjugate element which would preferably be made of optical quality material and an imaging lens. The imaging lens would be positioned between the window and the conjugate element at the appropriate distance from the window and the conjugate element so as to image the window onto the conjugate element.
One of the limitations of the above described system when used for such an application is that the depth-of-focus of the imaging lens that would typically be used for imaging the window onto the conjugate element would in most cases, not be as large as the maximum change in the thickness of the window over its entire area. As a result, only those portions of the window whose thickness changes fall within the depth-of-focus of the lens would be brought to focus on the conjugate element and corrected. Thus, the entire area of the window would, in effect not be corrected and thus not be useable.
As is known, the depth-of-focus of a lens is a finite value. This value is equal approximately to twice the mean wavelength of the light being transmitted through the lens multiplied the square of the F number of the lens. Thus, for a lens having an F number of 1 and being used with visible light, the depth-of-focus is about one micron. As is also known, the F number of a lens is equal to its focal length divided by its aperture diameter. As can thus be appreciated, if the wavelength (or wavelength band) of the light being transmitted remains fixed, the depth-of-focus of a lens can be increased by either increasing its focal length or by decreasing its aperture diameter or by changing both the focal length and the aperture diameter as appropriate. Unfortunately, increasing the focal length is not a practical way of increasing the depth-of-focus of an imaging lens when the lens is being used in an imaging system since such a change would cause an increase in the overall area occupied by the system. As can be appreciated, any change of this kind is very undesirable. On the other hand, decreasing the aperture size of the lens is also not a practical solution since this would decrease the amount of light that would be collected by the lens and in most instances a reduction in light collection capability is also very undesirable.
In U.S. Pat. No. 2,959,105 to K. Sayanagi there is disclosed an optical filter for use with an imaging lens. The optical filter comprises a support which is made of transparent material and includes a plurality of discrete mutually spaced spots of transparent film on one surface. The spots are formed by evaporation or other suitable means. Each spot is of a thickness of the order of the wavelength of the light passing through the filter and lens, is a desired shape and is of a material of a predetermined density. The spots of the plurality have a random distribution on the support and the ratio of the area of the transparent support covered by the spots to the portion of support area free of spots is unity. The thickness of the spots is such that the length of the transmission path of light through the spot-covered regions of the support is a half wavelength of the light longer than through the support free of spots.
It is an object of this invention to provide an optical filter for use in increasing the depth-of-focus of an imaging lens operating in broadband illumination while maintaining energy thru-put.
It is another object of this invention to provide an optical filter as described above which lowers the modulation transfer function of the imaging lens with which it is used.
It is still another object of this invention to provide an optical filter which is useful for eliminating aliasing in sampled imaging systems.